Electrical System

The XT500 is becoming a desirable bike to restore , collect and just to ride and it's popularity means there are many new and inexperienced owners. Most XT500s are now around 35 years old and they use traditional motorcycle electrical systems that are much different than newer bikes or automobiles. Although this collection of information is helpful for some, it has to go along with a good understanding of basic electrical laws (Ohm's Law) and understanding of basic electricity.


For many years I assumed that all XT500s had the same simple wiring harness and schematic as most models in North-America and Australia (permanently wired headlight and AC regulator) , and I thought the only exceptions were very early models that included a headlight on/off switch and a switched AC regulator that switched the regulator to the charging circuit when the headlight was off. As it turns out there are other different configurations depending on the year of manufacture and the region or country the bike was available.

The wiring systems can roughly be divided into models that had switchable headlights and models that had the headlight permanently wired to the engine stator with a single wire direct to the headlight dimmer switch.

In North America, XT500s with permanently wired headlights are the most common although it's possible some early 1975 or 1976 bikes may exist in North America with the earlier system. For owners of XT500s in North America, it's important to understand that for only1980 and 1981 models supplied to the US used a unique CDI ignition system and a sealed carburetor to meet EPA rules that mandated emmisions compliant ignition and carburetor. Even though the US models were also called XT500G and H and looked the same, they were different from Canadian and Australian bikes and the serial number prefixs were 3H6xxxx and 4R9xxxx.

At the time the XT was produced, Yamaha also supplied a sophisticated and proven 12 volt system with CDI ignition in the SR500 street-bike but the XT500 system was concieved as a much simpler system because the bike was viewed as a competition motorcycle that could be adapted to the street. For this requirement, the lighting and charging system as well as the accessories were designed to be lightweight, simple and removable without any alterations or impact to the running of the bike.

Generally, there are three separate main components of the XT500 systems, the ignition system, the headlight system, and a very simple low power battery charging system that supplies power to the accessories (brake light, horn, signal-lights and neutral indicator).

After many years it's sometimes hard to know exactly which electrical system applies to a certain bike, so it's important verify the system when trouble-shooting and not to follow the wrong diagram.

Here are two wiring diagrams that are reported to apply to European and UK motorcycles although I have no way of verifying which bikes use which diagrams.


XT500 C and D models with a 4-way switch that controls the Headlight:

Almost all of my experience has been with later models of XT500 (E, F, G and H) that only have 2 positions on the ignition switch. In these cases the ignition switch controls only the ignition and the battery powered accessories and the headlight is wired permanently on so it runs anytime the bike runs.

Earlier models had either a 3 or 4 way switch that allowed the headlight to be switched on and off. The positions of the switch were: 1 OFF....2 IGNITION AND ACCESSORIES ON and 3 IGNITION, ACCESSORIES AND HEADLIGHT ON.

On some models (typically the UK) there was a 4th position that controlled a small bulb in the headlight shell called a "Parking Light" as well as lighting the tailight for the purpose of marking the bikes position in areas where it was mandated that parked vehicles should have a light to make them easily visible.

Here is a wiring diagram of bikes with a 3 way and 4 way ignition switch:

At the upper left of the diagram you can see the extra Blue/White line to the tail-light as well as a RED/BLUE (lavender) line to the headlight shell parking light. In the diagram is a note "Park Lights (not all models)".


Here is a simplified version of the above diagram. The diagram shows the ignition switch in the headlight OFF position and the AC regulator line is switched to the charging circuit. With the headlight ON the regulator would be switched to control the headlight AC voltage. The AC going to the rectifier to charge the battery would be then become unregulated but the load of the headlight would reduce the charge current to an acceptable level.

Notice the Headlight on/off switch (three faint red circles) has three poles to switch the regulator from the headlight to the battery charging circuit and to turn the tailight on. In the larger diagrams there can also be provisions for a parking light and various power options with a 4 position key-switch.

2. XT500 models without a headlight switch:

In the 1970's, some countries began to mandate "daylight running lights" as a safety feature for motorcycles and XT500s for North American and Australia followed suit. For bikes like the XT500 that used AC headlight power it was convenient to simply delete the headlight switch which means the headlight is receives power anytime the engine runs. In this case it is logical to just wire the AC regulator permanently to the headlight circuit and simply delete the ability to switch the regulator from charging to the headlight.

Functionally these later models are the same as the models with a headlight on/off switch except they are permanently wired in the headlight ON state and the switching has been deleted. These XT models are the most familiar models supplied to Canada, Australia and the United States except for US models for 1980 and 1981 (see notes below).

Along with the elimination of a headlight on/off switch and with the AC regulator wired permanently to the headlight circuit these models generally had a much simpler wiring diagram.


Here is the lighting and charging system for later XT500s with a 2 position ignition switch and no provision to switch the headlight off. It's useful to compare this diagram with the one above.



3. The US XT500 Wiring Diagram for 1980 and 1981:

The XT500 G and H (1980, 1981) for North America and Australia came in two versions. The Canadian and Australian version Serial Numbers started with 3H7xxxx and 4T9xxxx and had normal XT500 points ignition, 4 magnet rotors and dog-bone generator coils and a very simple wiring harness. This model can be spotted by noting there is a points cover near the oil-filter cover on the right hand engine case.

On the American versions there is no points cover and no ignition drive train or points cam. These models were labeled 3H6xxxx and 4R9xxxx and were fitted with a specially designed 6 volt CDI system in order to meet emission requirements introduced in the US at the time. The new rules specified that the ignition system and carburetor would be adjusted for minimum emmisions and then non-adjustable by the user. These US versions for these two years are unique and different to ALL other XT500 versions including later models for the UK, Italy, Germany and France up to 1989 which retained points ignition.

The US XT500 with CDI ignition is more complex than the magneto/points system. Although the accessory circuits (headlight, and battery charging) operate the same they too are different physically to other XT500s.

Electrically the U.S. CDI version headlight, battery charging and accessory circuits operate the same as other XT500s but the components are much different. The stator coils are radial bobbin wound coils rather than "dog-bone" coils along with a dual-wound pickup coil. The rotor has 12 magnets rather than 4, the US CDI model has no points or point timing drive gears and the right side engine cover has no points access plate. These models also had much more complex wiring looms to accomodate the CDI ignition. As far as can be discovered, the only CDI versions of the XT500 were the 1980 and 1981 US models.

For XT500s with points ignition and no headlight off/on switch the following information applies:

The magneto/alternator has two small dog-bone coils, one is dedicated to the ignition and the other coil runs the charging system along with the headlight from a tap (an electrical connection in the middle of the coil) with a small AC voltage regulator. The tap reduces the voltage of the full coil which helps the regulator control the voltage to the headlight. The headlight isn't switched except for low and hi beam, the circuit is always ON and it runs anytime the bike is running.

The full winding of this same coil is used to charge the 6 volt battery through a simple "half-wave" rectifier (the diode sits just behind the headlight regulator to the left of the battery box) and charges the battery to run the neutral indicator, tail light, signal lights, horn and brake light.

Almost all of the electrical power of an XT500 is consumed by the headlight (over 4 amps), and supplying the headlight with AC direct from the magneto allows this heavy load to operate at almost 100% efficiency because there is no requirement to rectify the current and more important, it avoids the losses because of the reduced efficiency of normal battery charging and discharging processes.

This means the accessory system can be a very small and low powered and without a need for a charging regulator. The battery is used to store enough energy to smooth the diode output which is a pulsing positive wave, and then deliver smooth and reglated DC to operate the accessories. The storage of power in the battery is only asked to operate the accessories when the bike is running at idle or low speeds and is not generating much generator power. A battery in good shape won't run the accessories (especially the signal lights or horn) with the key on and the bike NOT running except for a very short time.

The system has some advantages over other systems, it's light, simple and easy to maintain and with both the headlight and the ignition operated directly from the magneto, your bike is not dependant on the battery charge to start and run with the headlight on.

Here's some basic notes to get you started:

1. If you are unfamiliar with bikes like the XT500 (magneto/ignition) bikes, make sure you understand that the IGNITION IS POWERED BY THE MAGNETO.....There is NO connection to the battery. The bike will run fine with a DEAD battery or WITHOUT a battery and there is never a time when your bike won't start because of a dead battery.

2. THERE WILL NOT BE A SPARK if the engine isn't rotated...Even if the key is on and the battery is fully charged. The rotor HAS to rotate for the ignition system to work.

3. THE SPARK INTENSITY depends on the speed of the rotor so if you test for a spark by kicking the kickstart-lever, the spark will be very weak. This does NOT mean the bike has a weak spark.

4. THE HEADLIGHT is a separate circuit also. It is driven directly from the magneto/generator, and has NO connection to the battery. The headlight lights anytime the engine is running and there is never a time when your headlight won't light because of a dead battery.

5. THE BATTERY is only there only to store a small amount of power for the accessories that are required to be highway legal and these are the brake-light, horn and signal lights, and neutral indicator.



3. XT500 E/F/G/H Wiring:

This is the full wiring diagram including ignition, for North American XT500 E,F,G and H. This diagram applies to all 1978, 79, 80 and 81 XT500s except for U.S.A. models of XT500 G and H with CDI ignition.




The ignition system starts with a separate generator coil in the magneto housing that is grounded on one end to the case. At the other end a wire runs to a plug on the oil tank and then to the spark plug coil. Near the 4 pin plug close to the battery case the wire is also hooked to the points and condensor (or capacitor) which are also grounded at the case. The the wire at the oil tank plug continues to the key switch and stop switch which simply grounds the line to stop the engine.


The ignition coil in the magneto alternator (orange circle lower right) is a simple iron core coil that feeds alternating current to the points in the black/white wire above. The same line also goes to the primary coil on the spark coil. When the points open the circuit, the magnetic field on the spark coil collapses and causes a high voltage to jump the spark plug gap. The ignition line (black/white) is extended to the key switch and run/stop switch which simply ground the line to stop the engine. This is a very basic and rock-reliable system but it requires proper timing and points gap.

The headlight system:

The headlight system has it's own separate circuit too. It starts with a center-tap (some of the windings) on the power coil in the magneto and the wire runs directly to the headlight dimmer switch. A connection to the wire takes a small amount of current to the panel lights. The dimmer switch just switches the current in the wire from the high to low beam in the headlight. Attached to this same line is another wire that goes to the AC regulator. The regulator is grounded to the frame and it limits the AC voltage in the line so the headlight and panel lights don't burn out. The XT500 AC regulator only controls the AC voltage to the headlight, it has no effect on the battery charging DC circuit.





Some owners try to measure the headlight voltage with a regular volt-ohm meter set to A/C and in this case the meter will read the RMS (the overall voltage multiplied by .707) of the wave as an AC waveform going from around -7 volts to +7 volts, so the meter will read about 10.3 volts AC. But a headlight filament can only "see" one part of the wave at any instant, so it is lit by either -7.3 volts or +7.3 volts at one time...not by the full excursion of the AC wave, so the meter will read much higher voltage than expected, as in the upper diagram.

In the lower diagram, if the headlight voltage is measured as DC, the meter will be affected only by the positive pulses (orange) pulses and because they are widespaced, the meter might interpret the voltage as lower than expected.

The Headlight Voltage Regulator:

The sole purpose of the regulator is to limit the voltage of the headlight supply line to protect the headlight and it not connected to, and has no effect whatever on charging or any other circuit on the bike except for the high-beam indicator and instrument lights that also take current from the headlight circuit. Except for the high/low beam switch the headlight has no switch and runs continuously when the engine is running.

Some care has to be taken to make sure the load of the headlight is present. If the headlight is burned out, removed or a switch is installed, the regulator will be overloaded with the full amount of current that would normally be consumed by the headlight so if you're going to run your XT500 without a headlight, unplug or remove the AC regulator and make sure the generator coil doesn't get accidentally grounded.


Charging system:

The battery charging system uses the same generator coil as the headlight except with all of the windings on the coil for more power. It also is a single wire that simply goes to the diode rectifier that converts the AC to pulsed DC, to the fuse. This wire then goes directly to the battery positive terminal.

The positive terminal of the battery and the red wire also connects to a wire that goes to the key-switch. This is the wire that supplies power from the battery to the accessories.

The charging system is very simple. The current from the charging coil goes through one wire to a single diode, through the fuse and then directly to the battery. Follow the blue and orange curcuit above. The blue represents raw AC from the coil, and the orange is DC direct to the battery.

You can test the system easily by starting the bike and testing the voltage directly across the battery terminals...all workshop manuals have a chart showing the voltage that should be present. Since this voltage is generated by a single coil and a single 1/2 wave rectifier there is not much to go wrong.

Measuring the output voltage of the charging system without the battery in place will indicate there is output but because the DC voltage is pulsing and is only there less than half the time, a meter will read the diode output as something less than the actually energy available. That means actually measuring the systems real voltage will require the battery to be in place.

Years ago I made a small chart measuring my voltage with a good operating battery:

Motor Off but key on 5.9 volts (taillight on)

1500 rpm


5.9 volts

2000 rpm


6.6 volts

3000 rpm


7.6 volts

Charging systems have to make more voltage than the battery size to accumulate a charge. A 6 volt battery with 6 volts applied will never charge enough to supply a useable current, so the system applies a heavier voltage (and current) as it turns faster. As you can see the system doesn't start to charge until 2000 rpm or so but the charge rate is still very small. The battery functions as both a regulator and a storage system in the hope that electrical output will roughly balance the charge rate. Mostly this is true but often the battery is overcharged slightly which burns off water rapidly.

The rectifier diode allows only the positive part of the AC to pass. That means the output to the battery is a series of positive pulses, and an important aspect is that when the diode blocks the negative part of the pulse, the diode's resistance is so high it is almost infinite. That means NO energy flows, NO heat is generated and the coil only has a "duty-cycle" of 50%. Because the system is totally shut off for half the time, it generates only half the heat and the stress is reduced considerably. Also, the charging current to the battery is low enough that no regulator is required.

Here's a diagram of both the headlight and the charging system and how they relate. Notice that although both systems are powered by the same coil, they are separate systems.


Most of the problems on XT500 systems are simply a bad battery. Often a rider will test the battery with a voltmeter and assume it's okay because the voltmeter shows 6 volts or even more. But even a worn out battery will test 6 volts, but won't have any capacity to deliver the voltage. A battery that is left uncharged in the cold will soon lose it's capacity to take a charge and freezing a discharged battery spells the end. Storing the bike should involve moving the battery to a warm place, and occasional recharges.

The simple flooded lead-acid battery has the unique capability of accepting (without damage) a wide range of fluctuating voltage where anything above the resting battery voltage (6.3 volts) results in a charging of the battery.

The output however, is held resonably close to 6.3 volts and this prevents any greater voltage from being delivered to the accessories. Even though it's a simple system it accomplishes not only storage of energy for times the alternator isn't producing power, but it regulates and smooths the output all with only 3 components, and it does all that without any extra energy loss or heat. It would be difficult to design a solid state system that has these features without extensive circuitry and sophistication.

Normally when an XT500 is running, the only steady current draw from the battery is 5.3 watts to the taillight (slightly more than a christmas tree bulb). Only occasionally indicator lights run (neutral indicator, signal light indicator) but they are only 3 watts. Signal lights,brake lights, and especially the horn consume far more current, but they are only on for very brief periods of time too.

This means that in normal use the flooded lead acid battery of an XT500 is overcharging almost continuously and this results in heat, gassing and loss of fluid. The big advantage of the ordinary flooded lead-acid battery is it will tolerate this overcharge by allowing gasses to exit the case rapidly, and water loss is easily detected and easily replaced.

Other, more modern types of batteries. For example gel-cells, AGM , nicads, Lithium Ion and other batteries shouldn't be used unless they have internal current limiting or charging regulators.....or if the XT500 system is changed to match the battery technology.

6 volts is a low voltage that can get stopped easily by a bit of corrosion or dirt on a connector. Open the headlight shell and unplug and reset the connectors now and then. The only real disadvantage to 6 volt systems compared to 12 volts is the fact that 6 volts has to have better and cleaner connections as it has less ability to "punch" through corroded connections.



It's a small battery. Usually you are expected to charge it at very low currents and I use 1/2 amp but you can use up to an amp or so. I use a small 6 volt car battery charger with a small signal light bulb in SERIES and I measure the current through an ammeter, with the light bulb in series it just happens to be 1/2 amp but anything up to 2 amps would be fine. Charging with a too high current will destroy the battery fast and in fact the battery is designed to have only slight discharges and mild charges. Running the battery dead flat and then full charging only works a few times before the battery is dead.

You can prolong the battery life a good bit by keeping the fluid level up to the line. XT500 batteries lose a lot of fluid (overcharging) and when the plates are exposed the battery dies. A monthly check is a good idea. Buy a liter of de-ionized water (for household irons) at the grocery store and use a small syringe to keep the level up. If you don't do this then budget for a new battery more often.

A battery that's cared for will last 2 or 3 years of use but it should be considered a consumable item and some people replace them every year. Even if the battery voltage measures 6 volts or better and the system isn't working it is a good bet the battery is toast and you will find that if your system has the proper signal light bulbs problems will go away with a new battery.




Signal Lights:

Probably the most confusing electrical problem on an XT500. The signal light flasher on 4T9 series XT (and many other models too) is a capacitor/resisitor type. It's the black rectangular plastic box beside the battery case with a 2 wire plug.

The basic circuit is very simple. 6 volts from the battery and key switch circuit is present anytime the key is ON. The circuit is activated when the signal light switch is makes contact with the left or right bulb circuit, like this:


Note the indicator bulb simply runs from one side to the other drawing power from the active side and then grounding through the inactive side filaments

A typical problem is the plugs and cover face up, and moisture can get in the box and corrode the contacts in the flasher. You can easily pry the circuit board out and use WD40 and a toothbrush to make them clean again, but over time the spring contacts can rust and cause open circuits. These too can be fixed by crimping the spring rivets with a pair of sharp side-cutters.

You can see rust on the relay contact springs of this 30 year old flasher relay.

The circuit is simply a resistor, capacitor and a relay. With the battery connected to "B" and the bulbs connected to "L" by means of the flasher switch, current flows through the relay and the large coil to light the bulbs. At the same time the resistor slowly charges the capacitor until the voltage is enough to pull in the relay opening the circuit and shutting off the bulbs. The capacitor drains and the relay closes again.

Although the large coil is also wrapped around the relay coil, it acts as a simple resistance "voltage divider" to force a small current flow to the resistor and capacitor. Without the small resistance of the "extra" windings the Battery connection "B" would short directly through the relay contacts to the "L" (lights) and the capacitor would be shorted and never charge and the relay wouldn't open.

On the left diagram, the battery power flows along the blue line to the right, through the relay and down the coiled resistor to the lights turning them on. But because the big coil has a slight resistance a small current is forced up the blue line to the left through the resistor, charging the capacitor.

When the capacitor collects enough energy, it pulls the relay open shutting off the signal light bulbs until the capacitor charge drops, then the relay closes and the cycle starts again.

The circuit is activated when the load bulbs are connected by the signal light switch and the flashing speed is controlled by the current through the bulbs. That means the bulbs have to be the right size to make the circuit work.

Here's the circuit path on the wiring diagram:



Note: this document is an ongoing search..if you spot an error, or can add to this knowledge, go to the homepage and scroll down to "Comments".